Analog multiplier circuits are used, for example in mass products of mobile radio, such as mobile telephones. They usually contain, both in the transmitting and in the receiving direction, an analog circuit which comprises all required circuit components for coupling the digital signal processing circuits to a radio interface. Depending on the modulation method, a carrier signal is modulated in the transmitting direction, and in the receiving direction, a received radio-frequency signal is combined with a heterodyne signal and translated into a low-frequency signal.
For the frequency conversion both in the transmitting direction and in the receiving direction, analog multiplier circuits are used in a so-called mixer mode. Further examples of applications for analog multiplier circuits are found in the splitting of the signals into a complex-valued signal with an in-phase component and a quadrature component normally used in modern mobile radio transmitters and receivers. This requires heterodyne or carrier signals which can be supplied to the multipliers, with a signal pair which has a precise phase displacement of 90° with respect to one another. Multiplier circuits, particularly those with similar signal inputs such as, for example, passive ring mixers, allow the phase displacement of 90° to be monitored in a particularly precise manner.
In the document Gray, Meyer: Analysis and Design of Analog Integrated Circuits, John Wiley & Sons, Third Edition 1993, ISBN 0-471-57495-3, a Gilbert multiplier cell constructed in bipolar circuit technology is specified in FIG. 10.9. This multiplier of the Gilbert type is an active multiplier which, however, has the disadvantage that the two signal inputs for supplying the signals to be multiplied are not electrically equivalent.
Such electrically non-equivalent signal inputs are shown, for example, in the document DE 236 50 59, compare there, for example, the interconnection of the signal sources V1, V2 with the differential amplifiers in FIG. 1. Both signal sources are coupled to the base terminals of the differential amplifiers via respective transistors. In this arrangement, however, the transistors allocated to source V2 are connected directly to the supply potential, whereas the transistors allocated to source V1 are connected to ground via resistors and a current source. Accordingly, a push-pull modulator with electrically non-equivalent signal inputs is shown.
Analog multiplier circuits in the fields of application supplied are subject to demands for ever lower supply voltage, little space requirement and producibility in inexpensive monolithic integration.
When the analog multipliers are used as frequency converters, that is to say as radio-frequency mixers, good linearity, little noise and high mixer gain is additionally required apart from the above-mentioned characteristics.
It is the object of the present invention to specify a multiplier circuit which, with high accuracy, can be used for monitoring the 90° phase difference of radio frequency signals.